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Updating comments to reflect the current state of the world after my recent changes to ownership structure and generally better describe what a GCStrategy is and how it works. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224086 91177308-0d34-0410-b5e6-96231b3b80d8
174 lines
7.3 KiB
C++
174 lines
7.3 KiB
C++
//===-- llvm/CodeGen/GCStrategy.h - Garbage collection ----------*- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// GCStrategy coordinates code generation algorithms and implements some itself
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// in order to generate code compatible with a target code generator as
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// specified in a function's 'gc' attribute. Algorithms are enabled by setting
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// flags in a subclass's constructor, and some virtual methods can be
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// overridden.
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//
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// GCStrategy is relevant for implementations using either gc.root or
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// gc.statepoint based lowering strategies, but is currently focused mostly on
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// options for gc.root. This will change over time.
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//
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// When requested by a subclass of GCStrategy, the gc.root implementation will
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// populate GCModuleInfo and GCFunctionInfo with that about each Function in
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// the Module that opts in to garbage collection. Specifically:
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//
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// - Safe points
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// Garbage collection is generally only possible at certain points in code.
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// GCStrategy can request that the collector insert such points:
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//
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// - At and after any call to a subroutine
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// - Before returning from the current function
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// - Before backwards branches (loops)
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//
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// - Roots
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// When a reference to a GC-allocated object exists on the stack, it must be
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// stored in an alloca registered with llvm.gcoot.
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//
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// This information can used to emit the metadata tables which are required by
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// the target garbage collector runtime.
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//
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// When used with gc.statepoint, information about safepoint and roots can be
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// found in the binary StackMap section after code generation. Safepoint
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// placement is currently the responsibility of the frontend, though late
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// insertion support is planned. gc.statepoint does not currently support
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// custom stack map formats; such can be generated by parsing the standard
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// stack map section if desired.
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//
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// The read and write barrier support can be used with either implementation.
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_CODEGEN_GCSTRATEGY_H
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#define LLVM_CODEGEN_GCSTRATEGY_H
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#include "llvm/CodeGen/GCMetadata.h"
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#include "llvm/CodeGen/MachineFunction.h"
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#include "llvm/Support/Registry.h"
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#include <string>
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namespace llvm {
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/// GCStrategy describes a garbage collector algorithm's code generation
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/// requirements, and provides overridable hooks for those needs which cannot
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/// be abstractly described. GCStrategy objects currently must be looked up
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/// through the GCModuleInfo analysis pass. They are owned by the analysis
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/// pass and recreated every time that pass is invalidated.
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class GCStrategy {
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private:
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std::string Name;
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friend class GCModuleInfo;
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protected:
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unsigned NeededSafePoints; ///< Bitmask of required safe points.
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bool CustomReadBarriers; ///< Default is to insert loads.
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bool CustomWriteBarriers; ///< Default is to insert stores.
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bool CustomRoots; ///< Default is to pass through to backend.
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bool CustomSafePoints; ///< Default is to use NeededSafePoints
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///< to find safe points.
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bool InitRoots; ///< If set, roots are nulled during lowering.
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bool UsesMetadata; ///< If set, backend must emit metadata tables.
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public:
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GCStrategy();
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virtual ~GCStrategy() {}
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/// Return the name of the GC strategy. This is the value of the collector
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/// name string specified on functions which use this strategy.
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const std::string &getName() const { return Name; }
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/// By default, write barriers are replaced with simple store
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/// instructions. If true, then performCustomLowering must instead lower
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/// them.
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bool customWriteBarrier() const { return CustomWriteBarriers; }
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/// By default, read barriers are replaced with simple load
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/// instructions. If true, then performCustomLowering must instead lower
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/// them.
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bool customReadBarrier() const { return CustomReadBarriers; }
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/** @name GCRoot Specific Properties
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* These properties and overrides only apply to collector strategies using
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* GCRoot.
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*/
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///@{
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/// True if safe points of any kind are required. By default, none are
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/// recorded.
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bool needsSafePoints() const {
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return CustomSafePoints || NeededSafePoints != 0;
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}
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/// True if the given kind of safe point is required. By default, none are
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/// recorded.
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bool needsSafePoint(GC::PointKind Kind) const {
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return (NeededSafePoints & 1 << Kind) != 0;
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}
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/// By default, roots are left for the code generator so it can generate a
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/// stack map. If true, then performCustomLowering must delete them.
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bool customRoots() const { return CustomRoots; }
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/// By default, the GC analysis will find safe points according to
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/// NeededSafePoints. If true, then findCustomSafePoints must create them.
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bool customSafePoints() const { return CustomSafePoints; }
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/// If set, gcroot intrinsics should initialize their allocas to null
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/// before the first use. This is necessary for most GCs and is enabled by
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/// default.
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bool initializeRoots() const { return InitRoots; }
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/// If set, appropriate metadata tables must be emitted by the back-end
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/// (assembler, JIT, or otherwise).
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bool usesMetadata() const { return UsesMetadata; }
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///@}
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/// initializeCustomLowering/performCustomLowering - If any of the actions
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/// are set to custom, performCustomLowering must be overriden to transform
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/// the corresponding actions to LLVM IR. initializeCustomLowering is
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/// optional to override. These are the only GCStrategy methods through
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/// which the LLVM IR can be modified. These methods apply mostly to
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/// gc.root based implementations, but can be overriden to provide custom
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/// barrier lowerings with gc.statepoint as well.
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///@{
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virtual bool initializeCustomLowering(Module &F) {
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// No changes made
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return false;
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}
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virtual bool performCustomLowering(Function &F) {
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llvm_unreachable("GCStrategy subclass specified a configuration which"
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"requires a custom lowering without providing one");
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}
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///@}
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/// Called if customSafepoints returns true, used only by gc.root
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/// implementations.
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virtual bool findCustomSafePoints(GCFunctionInfo& FI, MachineFunction& MF) {
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llvm_unreachable("GCStrategy subclass specified a configuration which"
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"requests custom safepoint identification without"
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"providing an implementation for such");
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}
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};
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/// Subclasses of GCStrategy are made available for use during compilation by
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/// adding them to the global GCRegistry. This can done either within the
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/// LLVM source tree or via a loadable plugin. An example registeration
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/// would be:
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/// static GCRegistry::Add<CustomGC> X("custom-name",
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/// "my custom supper fancy gc strategy");
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///
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/// Note that to use a custom GCMetadataPrinter w/gc.roots, you must also
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/// register your GCMetadataPrinter subclass with the
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/// GCMetadataPrinterRegistery as well.
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typedef Registry<GCStrategy> GCRegistry;
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}
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#endif
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